1. Field of the Invention
The present invention relates to a burst transfer system in a label multiplex system or a time-division multiplex system.
2. Description of the Related Art
There have been known, as data communication system, label multiplex systems such as ATM (Asynchronous Transfer Mode) or a packet transfer system, a time-division multiplex system that allocates time slots.
The ATM system as one of the label multiplex systems is that data is divided into 48 bytes, a header of 5 bytes is added to data of 48 bytes to form an ATM cell consisting of 53 bytes to be transferred. Those ATM cells are multiplexed in accordance with the amount of transmission of the ATM cells from a plurality of communication terminals and then transferred to a host device or a communication terminal at a receiving side through a communication network.
The ATM switch device constituting the communication network, in the case of transmitting the ATM cell from a plurality of input paths to the same output path, transfers the ATM cells after being temporally stored in a buffer memory upon the ATM cell collides with each other.
Because the capacity of the buffer memory in this case is limited, a large number of ATM cells toward the same output path are generated, resulting in an overflow of the buffer memory. In this case, the ATM cells which have overflown are abandoned.
For that reason, the communication network of the ATM system uses the communication control system which is called "flow control system".
In the flow control system, the communication terminals at the respective transmitting sides propose a band to be used for transmission to the communication network.
The communication network that receives the proposal from the communication terminal judges whether the proposed band is ensured or not, on the basis of the use state of the transmission path or the band or the like which has been reported from the communication terminal at other transmitting side.
The communication network, upon judging that the band reported from the communication terminal can be ensured, ensures the transfer of the ATM cell within the reported band. Further, the communication network, in the case that the ATM cell transmitted from the communication terminal exceeds the reported band, abandons the ATM cell transmitted over the band in priority.
On the other hand, the time-division multiplex system is a system in which a plurality of time slots that constitute a frame having a predetermined length are allocated to the communication terminal, respectively, and data from the respective communication terminals is inserted into the time slot which has been allocated to its communication terminal and transferred.
By the way, in the label multiplex system such as the ATM system, in the case where the communication terminal having an amount that exceeds the previously proposed band has been transferred (burst transfer), if another communication terminal also transfers the ATM cell having a relatively large amount, it causes a busy state such as the overflow of the buffer memory within a communication network, or the like to be generated, thereby abandoning the ATM cell. For that reason, the label multiplex system makes it difficult to conduct burst transfer.
Also, in the time-division multiplex system, if a plurality of continuous time slots are allocated to the communication terminal, the burst transfer can be conducted.
However, in the case where a plurality of switching units exist in the path of the burst transfer, it is necessary to synchronize the positions of the time slots that conduct the burst transfer between those switching units, thereby making it difficult to allocate the continuous time slots through a plurality of switching units.